In order to carry out the remote reading of meters, such as domestic gas meters, analogue information relating to drum position needs to be converted into electrical signals in the form of digital pulses. The analogue information can be converted into digital form by encoders operating according to either the incremental method or the absolute method. In both cases the digital information is fed to a microprocessor which in turn produces the appropriate reading at a remote point.
Incremental encoders convert the analogue information given by the rotation of the drum shafts into electrical pulses. One example of an incremental system consists of a magnetically operated switch which is activated by the movement of the drum representing, e.g., cubic feet of gas. Each activation increases a remote electronic counter by one digit, corresponding to an increase in gas consumption of one cubic foot. However, incremental encoders, although relatively inexpensive compared with absolute encoders, exhibit a number of undesirable features.
Firstly, they are prone to electrical noise, which can use extraneous pulses to be counted. These extra counts can only be reconciled after comparing the "true" meter reading with the stored information i.e., a visual inspection of the meter index reading given by the drums is required. Secondly, in the event of power failure, the encoder becomes inoperative and data is lost. Thirdly, in the event of a meter exchange, reconciliation of the stored information is required.
One solution to the above mentioned problems is to use absolute encoded indexes. Absolute encoders digitise the whole meter index reading i.e., each digit of each drum is encoded. The advantage of this method is that it is self-correcting, unlike incremental systems. Thus, the problem associated with electrical interference can be overcome by taking and comparing several readings. If these differ by, say, more than one digit, then additional readings can be taken. This can be repeated until the difference in readings is acceptable. Also, in the event of a power failure, an absolute encoded index maintains coding of the drums. On resumption of power, the meter can then be read with no loss of data. A further advantage of absolute encoders is that, in the event of meter exchanges, reconciliation of the meter reading is not required.
In summary, absolute encoders are self-correcting, whilst incremental encoders are not.
Although absolute encoders are known, existing types are based on the concept of employing one single rotatable contact associated with a series of discrete contacts, for each drum. With such an arrangement, it is possible for a rotatable contact to come to rest in the space between two adjacent fixed contacts, thereby giving a null reading and producing a resultant output error. Although arrangements have been proposed to overcome this disadvantage, these can still give rise to errors in the output readings obtained.